Patent GB-775832-A (GB)
Title: Improvements in or relating to picture communication systems
IPC: H04N7/01
Filed: 1955-09-23
Abstract: 775,832. Magnetic recording and reproducing apparatus. WESTERN ELECTRIC CO., Ltd. Sept. 23, 1955 [Sept. 30, 1954], No. 27201/55. Class 40 (2). [Also in Group XXXIX and XL (b)] In a combined telephone and television system, the television signals are reduced in bandwidth so as to be capable of transmission over a telephone channel by means of magnetictape recording and reproducing apparatuses at the transmitting and receiving stations, the signal due to only one frame in every n frames being recorded at the transmitter and read off at reduced band-width over the period of n frames, whilst the reverse process is employed at the receiver, the reduced band-width signals being recorded and read off n times as fast to restore the signal to its original form, the reading off operation being repeated n times. In effect the system effects a band-width reduction by transmitting only one frame in every n frames, the frame being restored to normal at the receiver and repeated n times so as to maintain the normal frame frequency. The drawing illustrates only the transmitting apparatus at a station A and the receiving apparatus at a station B. For normal two-way working the corresponding apparatus is added for transmission in the other direction. In addition to the telephone apparatus, which is conventional and not illustrated in detail, each station has a miniature camera cathode-ray tube 4, 4<SP>1</SP> and a picture-reproducing tube 5, 5<SP>1</SP>. Scanning proceeds at the rate of 20 frames per second, each frame comprising 60 lines, and operation at each station is controlled by a synchronous motor 16, 16<SP>1</SP> driving a shaft 15, 15<SP>1</SP> at 20 revs. per second. The motors are energized from a common 60 c/s supply, a phase shifter 85 being included to permit accurate phasing of the operation at the two stations. At the transmitting station, station A, the horizontal and vertical scanning is controlled by generators 8 and 9, synchronization being effected by signals induced in magnetic heads 12 and 14 in response to the rotation of metallic discs 11 and 13 carried by shaft 15 and formed around their circumferences respectively with one tooth and 60 teeth. The camera signals appear on lead 17 and are applied via a contact system 20 to the recording head 33 in a magnetic-tape recorder and reproducer. Contact 20 is operated by a cam 21 driven via 20 : 1 reduction gearing from shaft 15 the arrangement being such that the contact is closed for 1/20 th sec. in each revolution whereby signals due to one frame only in each twenty pass to head 33. In the magnetic tape apparatus, the recording head 33 is carried by shaft 15 so as to rotate at 20 revs. per second and moves around a nearby circular loop of tape 31, 34 is a stationary pick-up head. The tape is traversed by driver roller 26 and the radii of roller 26 and loop 31 are selected so that the relative speed of the tape with respect to the magnetic heads 33 and 34 is precisely twenty times greater in the case of head 33. Thus the single television frame lasting 1/20 th second applied to head 33 is recorded and subsequently read off by head 34 at 1/20 th the speed so as to occupy 20 frames or 1 second. The resulting signal of reduced bandwidth (1600 c/s for the scanning standards specified) is capable of transmission within the central 2000 c/s portion of a 4000 c/s telephone channel 45 and is applied to the channel by conventional single side-band or vestigial sideband techniques by means of modulator 37 and carrier generator 36. Synchronizing signals derived via amplifiers 41 and 42 may be added to the recorded and subsequently transmitted signals by closing switch 43. At the receiving station, station B, a reverse recording and reproducing operation is carried out to restore the signal to its original form, the signal being recorded on a moving magnetic tape by a stationary head, the operation taking one second and then the tape being stopped and the signal read off by a moving head in 1/20 th second, the reading off process being repeated twenty times. Two tapes 53 and 54 are necessary, each with its stationary recording head 55, 56 and moving pick-up head 57, 58, one tape being used for recording whilst signals are read off from the other. Switching between the tapes is effected by switch mechanism 70 actuated by a cam 65 rotated via a 40 : 1 reduction gear at half-frame frequency. The tape traverses are clutched into and out of operation by electromagnet 71 and 74 actuating idler rollers 72 and 75 co-operating with the tape drive rollers 77 and 78. The received channel signals are demodulated at 50, 51, and the restored signals are applied to the reproducing cathode-ray tube via lead 80. Synchronization may be effected as at station A by discs 11<SP>1</SP>, 13<SP>1</SP> on the motor shaft, or by means of synchronizing signals sent from station A if switch 43 at that station is closed. In this event switches 83 and 84 are operated to connect scanning generators 8<SP>1</SP> and 9<SP>1</SP> to sync. separators 81 and 82 connected to signal lead 80.